Method of preheating combustion supporting air for steam generating plants



Jan. 18, 1955 2,699,758

, D. DALIN METHOD OF PREHEATING COMBUSTION SUPPORTING AIR FOR STEAMGENERATING PLANTS Filed July 11, 1950 2 Sheets-Sheet l Jan. 18, 1955 D.DALIN METHOD OF PREHEATING COMBUSTION SUPPORTING AIR FOR STEAMGENERATING PLANTS 2 Sheets-Sheet 2 Filed July 11, 1950 United StatesPatent METHOD OF PREHEATING COMBUSTION SUP- ;OEEING AIR FOR STEAMGENERATING L TS David Dalin, Stenlrullen, Ronninge, Sweden, assignor toA/B Svenska Maskinverken, Sodertalje, Sweden, a corporation of SwedenApplication July 11, 1950, Serial No. 173,068

Claims priority, application Sweden February 2, 1946 13 Claims. (Cl.122--1) This application is a continuation in part of my copendingapplication, Serial No. 725,493, filed January 31, 1947, now abandoned;and the invention relates to steam generating plants and has moreparticular ref erence to a method and apparatus for improving combustionin the furnaces of steam boilers by preheating the combustion supportingair to relatively high temperatures using the flue gases as a source ofheat for this purpose.

The conventional air preheater in which the combustion supporting air isheated by direct transfer of heat from the flue gases through the wallsof tubular air passages which comprise the heat exchanger of thepreheater structure has serious disadvantages, especially when fuel ofpoor quality such as pulverized coal, sulphite lye and other so-calledpulver-rich fuels, are burned. With such fuels it is difficult to obtaincomplete combustion. Hence, the flue gases contain large quantities ofsolid and gaseous substances prone to adhere to any surface with whichthey come in contact and if the flue gases condense on these surfaces,destructive corrosion results due to the ensuing chemical reactionbetween the condensate and the foreign substances.

To preclude such destructive corrosion and to main' tain efficientoperation, the heating surfaces must be frequently cleaned but since thetubular air passages of the conventional air preheater are closelyspaced, fixed structures, they are very difficult to keep clean.

While these disadvantages of the conventional air preheater have beenappreciated in the past and attempts have been made to overcome them, asin the patent to Roe, No. 1,833,130, issued November 24, 1931, whereinan intermediate heat vehicle or medium is circulated in heat exchangerelation with the hot flue gases and the air to be heated, all priorefforts to solve the problem have fallen short of reaching their goal,especially where low quality fuel is being burned.

The present invention, like the patent to Roe, employs an intermediateheat vehicle and thus enables the use of vibratable heating surfaces inthe flue gas duct so that the problem of keeping the heating surfacesclean is greatly minimized. In other words, the heating surfaces overwhich the flue gases flow may be serpentine coils supported as disclosedin the copending application to David Dalin, Serial No. 622,832, filedOctober 17, 1945, now Patent No. 2,550,676, so that the tubes may bereadily vibrated to dislodge foreign matter accumulating thereon. v

However, merely utilizing an intermediate heat vehicle and employingvibratable heating surfaces in the flue gas duct is not sufficient. Toachieve its primary purpose of improving the combustion of poor qualityfuel, the present invention provides for heating the combustion air to avery high temperature and with a view toward achieving efficiency theinvention further contemplates the abstraction of as much as possible ofheat energy from the flue gases. These objectives are accomplishedthrough a novel two-stage heating of the combustion air, the air beingpreheated by heat abstracted from the flue gases at the coolest portionof the flue gas duct and being further heated by heat abstracted from amuch hotter zone of the flue gas duct spaced from the firstheatabstraction zone by an intermediate heat absorbing unit orexchanger.

The invention contemplates still another important point and that is thecontrol of the temperature of the heating surfaces in the flue gas ductand especially those which provide the heat for the first preheatingstage to assure that the temperature of these surfaces will at all timesbe higher than the dew point of the flue gases flowing thereover.

In summation therefore, the purpose and objective of this invention isto provide a method and apparatus for utilizing the heat contained inthe flue gases to preheat the combustion air to a temperature highenough to enable poor quality fuel and fuel having a high moisturecontent to be successfully burned, and to achieve this heat transferfrom the flue gases to the combustion air by the boiler fluid containedin the steam generating plant in a manner which assures the abstractionof the maximum amount of heat energy from the flue gases all withoutdanger of condensation of the flue gases upon the heat abstractingsurfaces over which they flow, notwithstanding a possible high S02 andS03 content in the gases, the presence of which tends to lower the dewpoint of the gases and, of course, results in a very serious corrosionproblem.

With the above and other objects in view, which will appear as thedescription proceeds, this invention resides in the novel method andsequence: of operations substantially as hereinafter described and moreparticularly defined by the appended claims, it being understood thatsuch changes in the precise embodiment of the hereindisclosed inventionmay be made as come within the scope of the claims.

The accompanying drawings illustrate two complete examples of the methodof this invention propounded in accordance with the best procedure sofar devised for the practical application of the principles thereof, andin which:

Figure 1 is a diagrammatic view illustrating the preferred method forpreheating the combustion air flowing into the furnace of a steamgenerating plant; and

Figure 2 is a diagrammatic view similar to Figure 1 but illustrating aslightly modified method.

Referring now more particularly to the accompanying drawings in whichlike numerals indicate like parts, the numeral 5 designates the furnaceof a steam generating plant equipped with steam generating surfaces orboiler tubing (not shown) and having the customary steam dome 6associated therewith.

The furnace is provided with a grate 7 in its lower regions to support abed of burning fuel, and has an inlet 8 in the side of the furnace adistance above the grate providing for the charging of wood or othersolid fuel into the furnace to be burned upon the grate 7. A pair ofinlet ports 9 in the side of the furnace a suitable distance above thegrate provide for the injection of pulverized coal or other pulver-richfuel into the interior of the furnace, while additional inlet ports 10beneath the pulverized coal ports provide for injecting sulphite lye orthe like into the interior of the furnace.

Primary combustion air is supplied to the furnace through a duct 12opening to the interior of the furnace through a port 13 in the sidewallof the furnace at a zone adjacent to the grate 7. A blower 14 adjacentto the inlet of the duct is provided to force-feed the air into thefurnace.

The flue gases emanating from the furnace are conducted away from theupper part thereof through a flue gas passage or duct 15, and in theirpassage through this duct flow across a plurality of heat exchangers 16through which boiler fluid is circulated. Thus a primary superheater 17is located in the hottest portion of the flue gas passage adjacent tothe inlet thereof and a secondary superheater 18 is located directlydownstream therefrom. Next in line is an economizer 19 23d beyond theeconomizer is a primary heat exchanger It is important to note that theeconomizer 19 is positioned between the primary heat exchanger 20 andthe superheaters 17-18. This assures a definite temperature differentialbetween the two zones from which the superheaters and the primary heatexchanger derive their heat, and since the primary heat exchanger 20 islocated farthest downstream the flue gases flowing thereover are appreciably cooler than those flowing over the superheaters but still containmuch heat energy, which should be abstracted if waste is to be avoided.

The primary heat exchanger comprises a group of coils having theiroppositeends connected to inlet and outlet headers 21 and-'22,respectively, and suspended to bereadily vibrated. An-y suitable mannerof suspending the coils maybe employed but that shown and described inthe aforesaid pending application, now Patent No. 2,550,676, ispreferable. The *boiler fluid circulated through and heated in the coils.of the primary heat .exchanger 20, in accordance with the method ofthis inventionQis in a liquid state (water) and constitutes anintermediate heat vehicle or medium by which heat is indirectlytransmitted-from the flue gases to the combustion air flowing hrough h1'2- .FQf hi pu pose secondary h a e hang d! is p s tioned insidetheduct 12 to have heated fiuid from the primary heat exchanger 20circulated therethrough. The secondary heat exchanger 24 maybe of anysuitable const u ti but u t mar y comprise a er o pip of tubesconnecting inlet and outlet headers 25 and 26, respectively. The inletheader 25 is connected with the outlet header 22 of the primary heatexchanger 26 by a ee 1111921. n th ou le h ader .26 of h s cond ry heatexchanger is connected with an accumulator 28 through a pipe l ne .29- The acc mula or h r P p 39 'leadingfrorn a point below its normal waterlevel to the inlet of a pump 31, the discharge end of which is connectedwith the inlet header 21of the primary heat exchangerlt), through a pipe32.

The pump 31 thus circulatesthe liquid boiler fluid heatedin theprirnaryheat exchanger 20 through the secondary heat exchanger 24 tothereby effect an indirect transfer of hea from th was e fl gas s t thecombustion air to preheat, the lat er.

' While it is desirable to abstract as much as possible of the heat enery f om the flu es, i is impor nt h the temperature of the liquidcirculating through the primary heat exchang r, and henc the heati g rfas here f, be kept safely a v h d point of the the gases flowingthereover. To this end a bypass pipe 33 connects the outlet side of theprimary heat exchanger, specifically the ipe Withthe inlet of the pumpthrough a thermostatically controlled valve 34which controls flowthrough thepipe 3 and, has its thermally responsive control e1cmentj35located in the pipe 32 between the discharge ,end .of the pump 31 andthe inlet header 21 of the primary heat exchanger 2%). Whenever thetemperature of the fluid returning to the primary heat exchanger isbelow a predetermined value, the thermally responsive element 35functions to open the valve .34 allowing heated fluid in the pipe, 27 tobypass the secondary heat exchanger 2'4 and flow directly to the inletof the pump to mix with the cooler fluid from the accumulator andthereby raise the temperature of the fluidreturningto the primary heatexchanger. In this manner the temperature of. the fluid entering the.primary heat exchanger 20, and consequently its heating surfaces,ismaintained above the dew ,point of the flue gases flowing thereover.

A pipeline- 36 connecting the accumulator 2,8 with the steam dome 6serves, through condensation. of steam in the line 36, to maintain anadequate supply of boiler fluid in the air preheating system justdescribed and also affords a .relieffor the circuit which is otherwiseclosed.

The'ternperature of the air which is preheated by passage over thesecondary heat exchanger 24 may be high enough for use with furnacesburning relatively high grade fuels, but it is too low to overcome theobjection of incomplete combustion with the lower grade fuels mentionedhereinbefore and fuels. of high moisture content. Thus, the method ofthis invention, contemplates further heating of the airflowing throughthe duct 12 to a .tem-

perature. of atleast between 200 to 300 C. For this purpose .a secondstage ,air preheater 40 is located in the duct 12 near the furnace toprovide-second stage heating oftheair. This airheater, like the firststage heater (the heat exchanger 24), derives its heat indirectly fromthe flue gases, but through the ,rnedium of the much hotter superheatedsteam coming from the superheater 1,7.

structurally the second stage air heater 40 may be of any suitabledesign, but like the heat exchanger 24 preferably comprises a group oftubes .or coils having one end connected. to a comrnoninlet header 41and their opposite ends connected with a common outlet header 542. Apipe ine 4.3dilz etly connects th inle head r 4. with th ou let header 4f he primary superhea er .17 sothat superheated steam from the primarysuperheater flows by its own pressure through the coils of the heater40.

The inlet header 45 of the primary superheater 17 is connected by meansof a pipe 46 with the steam dome 6 from which the superheater issupplied with steam; and while the superheated .steam naturally losesboth pressure and temperature in its circulation through the air heater49, this loss is restored by reheating the steam in the secondarysuperheater 18 and to this end the outlet header 42 of the air heater 40is connected by means of a pipe line 48 with the inlet header 49 of thesecondary superheater 13. The outlet header 50 of the secondarysuperheater may be connected to one or more locations at which thesuperheated steam is to be used.

The modified embodiment of the invention illustrated in Figure 2 differsfrom that of Figure 1 mainly in that the superheated steam used. in thesecond stage of combustion air preheating is allowed to condense as itcirculates through the ,air heater .40. Hence, :the superheated steamgivesoif its heat of evaporation to thceonibust'ion air and therebyraises the temperature ofwthe air :to a suitably highdegree. Also, onlypartofathe superheated steam issuing from the primary superheater 1'7 isutilized for the heating .of the combustionair. The outlet'header 44 ofthe superheater '17 in addition to being connected with the inlet header41 of the heat exchanger .40 has a line 43 leading therefrom to one ormore points at which the superheated steam is to be utilized. Thecondensate issuing from the outlet header. 42 of the heat exchanger 40is led to .a reservoir 52 by means .of a-pipe 54. A purnp 55' and a pipeline 56 leading from the discharge side of the pump feeds the condensatefrom theaccumulator to the inlet header S7 of the economizer 19located-inthe flue gas duct between the primary heat exchanger 20 and asteaming section 18 which corresponds in location to the secondary steamsuperheater 1.8 vof the Figure leni-- bodiment. .In .its passage throughthe economizer 19 :the condensate is, of course, reheated before ,it isreturned to .the steam dome through a pipe line 518 leading from theoutlet header 59 of the .economizer.

The steaming coils 18 have boilerfluid circulated therethrough directlyfrom and back to the steam dome by means of a pump 60 and feed andreturn lines 61' and 62, respectively,.connectedwith the inlet andoutlet headers 63 and 64', ofthe steaming coils 18'.

The circulatory system forythe liquid heatvehicle by which the initialor first stage air preheating is effected is similar to that shown inFigure -1 except that the accumulator 28 is not connected with the steamdome, but instead a safety valve 65 is mounted in the top of theaccumulator to relieve excessive pressure in the system. Since thesystem is closed except for the possible opening of :the relief valve,replenishment of the liquid circulating therein is seldom necessary butwhen required, maybe fed thereto through a feed line 66 provided with asuitable valve .67 and adapted to be connected with any suitable sourceof heated water as, for instance, the feed water in the steamdome.

From the foregoing description taken in connection with theaccompanyingdrawings, it--will be readily apparent;to those skilled in:the art that the method of =this invention enables the heating of thecombustion air to the high emperature required to successfully burn poorquality fuel and fuel of high moisture content and that it alsoabstracts the maximum heat energy from the flue gases withoutdangerofdestructive corrosion of the heat-- ing surfaces over which the:flue gases flow which would result if condensation of .the flue gases.on these surfaces were permitted.

The explanation for the success .of this invention perhaps will he morereadily appreciated from alconsideration of the following temperaturevalues whichuobtain in a typical installation of this invention. Theflue gas temperature in such a typical installation at entrance of. theflue gasduct is about 1000 C. and the temperature of the superheatedsteamissuing from the primary superheater 17 is about 400 :C. :In viewof this high temperature of Ihesunerhetaed steam, t Combustion airalready preheatedv in'its passage over the first stage air heater,namelytheheat exchanger 24, is readilyheated at thesecond stage airpreheater A0 to a temperature sufliciently highas to :assure rapidandcomplete-combustion of practically any ;fuel :that might be chargedinto. the=furnace r I I For the fuels herein mentioned, the combustionair must be heated to between 200 C. and 300 C. and this, as stated, isreadily accomplished. In raising the temperature of the combustion airto this value the temperature of the superheated steam isproportionately lowered and in the Figure 1 embodiment is reheated toabout 425 C. or 450 C. in its passage through the secondary superheater18. In the Figure 2 embodiment of the invention the superheated steam isallowed to condense as it heats the combustion air to thus furtherassure raising the temperature of the air to the desired value by virtueof the steam giving up its heat of evaporation.

In the superheating and reheating of the steam in the Figure lembodiment and in the superheating of the steam and the heating of theboiler fluid in the section 18 in the Figure 2 embodiment, the flue gastemperature is reduced from its initial 1000 C. to approximately 650 C.,and in its passage over the economizer the flue gas temperature isfurther reduced to between 250 C. and 300 C. Hence, the temperature ofthe flue gases which provide the heat source for the first stage of theair preheating is considerably less than that which prevails in the zonewhich affords the heat for the second stage of preheating of the air;and finally in its passage across the primary heat exchanger 20 thetemperature of the combustion air is further reduced to approximately125 C. From this it is evident that maximum heat energy is abstractedfrom the flue gases and that at the same time the temperature of thecombustion air is raised to its desired high value.

It will also be apparent that the spacing of the two zones which providethe heat sources for the first and second stage of air preheating, bymeans of the intermediate heat exchange surfaces, constitutes animportant feature of the invention.

What I claim as my invention is:

l. The method of preheating the combustion air being fed to the furnaceof a steam generating plant having a flue gas duct through which hotflue gases discharge from the furnace, which comprises: withdrawingsteam from the plant; superheating the steam so withdrawn by circulatingit in indirect heat exchange relation with the flue gases flowing in arelatively hot zone of the flue gas duct; circulating the superheatedsteam in indirect heat exchange relation with the combustion air beingfed to the furnace to heat the same; abstracting heat energy from theflue gases leaving said relatively hot zone to thereby materially reducethe temperature of the flue gases below that prevailing in saidrelatively hot zone; and circulating a liquid heat exchange medium inindirect heat exchange relation both with the flue gases flowing througha zone of the flue gas duct downstream from that zone at which said heatabstraction occurs and with the combustion air before said combustionair is heated by the superheated steam in the manner stated to therebypre-v heat the air and further reduce the temperature of the flue gases,whereby the combustion air is heated to a high degree while at the sametime the temperature of the flue gases is reduced to a value below thatwhich could be attained by the abstraction of heat therefrom by thesteam or the liquid heat exchange medium alone.

2. The method of claim 1 further characterized by regulating the rate atwhich said liquid heat exchange medium is circulated in heat exchangerelation with the combustion air in accordance with the temperature ofsaid liquid heat transfer medium as it begins its circulation inindirect heat exchange relation with the flue gases to thereby precludecondensation of the flue gases as they flow in indirect heat exchangerelation with said liquid heat transfer medium.

3. The method of preheating air for supporting combustion in the furnaceof a steam generating plant having a flue gas duct through which hotflue gases discharge from the furnace, said method comprising:continuously circulating some of the liquid boiler fluid of the plantserially in indirect heat exchange relation both with the flue gases ina zone of the flue gas duct remote from the combustion zone of thefurnace and with the air for combustion to thereby effect first stagepreheating of said air; withdrawing steam from the plant andsuperheating it by heat exchange with the flue gases in a zone of theflue gas duct at which the minimum temperature is considerably higherthan the maximum temperature in the zone where the said liquid boilerfluid is heated; and circulating the superheated steam in indirect heatexchange relation with the combustion air already heated by the liquidboiler fluid to thus eifect second stage preheating of the air.

4. The method set forth in claim 3 further characterized by circulatingthe steam cooled by such heat exchange, in heat exchange relation withthe flue gases in a zone of the flue gas duct intermediate said othertwo zones.

5. The method of preheating the combustion air being fed to the furnaceof a steam generating plant having a flue gas passage through which hotflue gases discharge from the furnace, said method comprising:continuously and serially circulating some of the liquid boiler fluidfrom the plant in indirect heat exchange relation both with the fluegases in a zone of the flue gas passage remote from the combustion zoneof the furnace and with the combustion air being fed to the furnace tothereby eifect first stage preheating of said air; withdrawing steamfrom the plant and superheating it by circulating it in indirect heatexchange relation with the flue gases: in a zone of the flue gas passageat which the minimum temperature of the gases is considerably higherthan the maximum temperature of the gases at said other zone;circulating a portion of the steam so superheated in. indirect heatexchange relation with the preheated combustion air to eflect secondstage preheating of said air and to condense the steam; circulating thecondensed steam in indirect heat exchange relation with the flue gasesin a zone of the flue gas passage between said two other zones toregenerate it into steam; returning the steam so regenerated back to thesteam plant; and conducting the remaining portion of the superheatedsteam to a point at which it may be utilized as desired.

- 6. The method of preheating air for supporting combustion in thefurnace of a steam generating plant having a flue gas passage throughwhich hot combustion gases discharge from the furnace, said methodcomprising: continuously and serially circulating some of the liquidboiler fluid from the plant in indirect heat exchange relation both withflue gases in a zone of the flue gas passage remote from the combustionzone of the furnace and with air for combustion to thereby'effect firststage preheating of said air; bypassing some of the boiler fluid whichwould otherwise circulate in said heat exchange relation with thecombustion air and returning it into heat exchange relation with theflue gases in said zone to thereby maintain the temperature of saidliquid boiler fluid above the dew point of the flue gases flowingthrough said zone; withdrawing steam from the plant; superheating thesteam so withdrawn by circulating it in heat exchange relation with theflue gases in a zone of the flue gas passage at which the minimumtemperature is considerably hotter than the maximum temperature at saidfirst zone; and circulating the superheated steam in indirect heatexchange relation with the combustion air already heated by the liquidboiler fluid to thereby effect second stage preheating of the air.

7. The method of preheating air for supporting combustion in the furnaceof a steam generating plant having a flue gas passage through which hotcombustion gases discharge from the furnace, said method comprising:continuously circulating some of the boiler water from the plant inindirect heat exchange relation both with the flue gases in a zone ofthe flue gas passage remote from the combustion zone of the furnace andwith air for combustion to thereby effect first stage preheating of saidair; recirculating some of said heated boiler water directly back intoheat exchange relation with the fiue gases in said zone without firstcirculating it in heat exchange relation with the combustion air;controlling the quantity of boiler water being so recirculated; mixingthe recirculated boiler water with the continuously circulating boilerWater to maintain the temperature of the water passing in heat exchangerelation with the flue gases above the dew point of the flue gases insaid zone; withdrawing steam from the boiler plant; spperheating thesteam so withdrawn by circulating it in heat exchange relation with theflue gases in a relatively hot zone of the flue gas passage; andcirculating the superheated steam in indirect heat exchange relationwith the combustion air already heated by the boiler water to therebyeffect second stage preheating of the air.

8. The method of preheating air for supporting combustion in the furnaceof a steam generating plant having a flue gas passage through which hotcombustion 9 10 References Cited in the file of this patent 2,170,345Bailey et a1 Aug. 22, 1939 UNITED STATES PATENTS 2,424,587 Smlth et a1.July 29, 1947 1,741,567 Heaton Dec. 31, 1929 FOREIGN PATENTS 1,753,363Coghlan et a1 Apr. 8, 1930 5 299,436 Great Britain 1,819,174 JacobusAug. 18, 1931 (Not accepted but printed in 1930) 1,833,130 Roe Nov. 24,1931 729,409 France July 23, 1932 1,837,713 Jacobus Dec. 22, 1931575,509 Great Britain Feb. 21, 1946 1,840,836 Davis Jan. 12, 1932629,298 Great Britain Sept. 16, 1949 1,975,519 Rudorfi Oct. 2, 1934 10633,361 Great Britain Dec. 12, 1949

